Batteries can be broadly classified into primary and secondary batteries. Primary batteries, also referred to as disposable batteries, are intended to be used until depleted, after which they are simply replaced with one or more new batteries. Secondary batteries, more commonly referred to as rechargeable batteries, are capable of being repeatedly recharged and reused, therefore offering economic, environmental and ease-of-use benefits compared to a disposable battery.
Although rechargeable batteries provide a much longer service life than disposable batteries, their service life is not unlimited. There are a number of factors that limit battery service life, including; (i) the number of recharging cycles the battery has been subjected to, (ii) the rate of charging (i.e., slow trickle charge versus fast charge), (iii) the level of charging (i.e., 75% of full charge, full charge, over-charged, etc.), (iv) the level of discharge prior to charging (i.e., completely depleted, still charged to a low level, etc.), (v) the storage temperature of the battery during non-use, and (vi) the temperature of the battery during use. Additionally, battery internal mechanical and chemical instability can adversely affect battery service life.
In general, the battery chemistries used in secondary cells are less stable than those used in primary cells. As a result, secondary cells often require special handling during fabrication. For example, lithium-ion batteries are typically manufactured in humidity-controlled, dry rooms and sealed to minimize subsequent water contamination. Batteries may also be manufactured in an inert atmosphere, thereby preventing cell contamination from any of a variety of reactant gases.
Batteries are sealed to prevent leakage and/or contamination from water, oxygen, carbon dioxide, or other materials. Unfortunately, battery seals are imperfect, thereby allowing gradual contamination and degradation of the batteries. One approach to overcoming this problem is to improve the battery seals. For example, U.S. Patent Application Publication No. 2003/0096162 discloses a hermetic seal that is compatible with corrosive electrolytes such as the lithium-ion electrolyte used in a lithium cell. Although improved battery seals offer one approach to overcoming contamination issues, this approach typically requires different solutions depending upon the cell chemistry and geometry in question.
Another approach to preventing cell contamination from water is to control the relative humidity within the battery pack itself, for example using passive desiccant bags. This approach is described in co-pending U.S. patent application Ser. No. 12/386,684. Unfortunately, as sources of moisture production within the battery pack persist throughout the life of the pack, these types of passive desiccant bags eventually reach a saturation point at which point they can no longer absorb additional moisture. As a result, once saturation is reached, the desiccant bags must be replaced or the relative humidity within the pack will rise to a level that may result in dewing events. Desiccant bag replacement introduces undesirable service intervals throughout the life of the battery pack in order to replace the bags as they become saturated.
Accordingly, what is needed is a maintenance-free solution to battery pack humidity control which is operable throughout the service life of the battery pack. The present invention provides such a system.